If you dream of decapitation, it means you lost your head, literally. The dream analysis of losing your head means you lost your rationale. May be you did] not act characteristically as per your norm. The dream imagery of a headless person or creature, is a metaphor. A living thing without a head & brains, has no organic matter for its thinking process.
The dream analysis would require you to reflect on your present context in real life. Are you stressed? Do you feel so pressurized that you feel paralyzed to think and act? Do you ask yourself, where is your head? Perhaps your subconscious brain has created this dream about a headless creature. maybe the message is for you to realize what is happening and get some help to manage the situation.
Meanings of dreaming being killed by beheading: